Estimated Delivery Date Calculator

Frequently Asked Questions

What is the estimated delivery date (EDD)?

The estimated delivery date (EDD), also known as the estimated due date (EDD), is the approximate date when a pregnancy is expected to reach 40 weeks (full term). It is calculated using Naegele's rule: EDD = LMP + 280 days (40 weeks). Only about 4–5% of women deliver exactly on their due date.

What is Naegele's rule for calculating EDD?

Naegele's rule is the standard method for calculating EDD: take the first day of the last menstrual period (LMP), add 280 days (40 weeks), or equivalently add 9 months and 7 days. For example, LMP of January 1 gives EDD of October 8. The rule was developed by German obstetrician Franz Naegele in 1838.

How accurate is Naegele's rule?

Naegele's rule has an accuracy of approximately ±1–2 weeks in women with regular 28-day cycles. The accuracy decreases with irregular cycles, uncertain LMP dates, or recent oral contraceptive use. First-trimester ultrasound provides the most accurate dating, with ±5–7 day precision.

What is the Parikh formula for EDD?

The Parikh formula adjusts Naegele's rule for cycle length: EDD = LMP + 280 days – 21 days + actual cycle length. For a 35-day cycle: EDD = LMP + 280 – 21 + 35 = LMP + 294 days. This formula improves accuracy for women with long or irregular cycles.

What is the difference between LMP-based and ultrasound-based EDD?

LMP-based EDD uses Naegele's rule from the first day of LMP. Ultrasound-based EDD uses fetal measurements (crown-rump length in the first trimester) to estimate gestational age and calculate EDD. When the two differ by more than 5–7 days in the first trimester, ultrasound dating is typically used.

When should the due date be changed based on ultrasound?

ACOG guidelines recommend changing the EDD based on first-trimester ultrasound when: (1) CRL measurement differs by more than 5–7 days from LMP-based dating, or (2) LMP is uncertain or unknown. For second-trimester ultrasound, a difference of 10–14 days prompts redating.

What is the typical range for a normal delivery?

Full-term pregnancy is 37 to 42 weeks. Early term is 37–38 weeks, full term is 39–40 weeks, late term is 41 weeks, and postterm is 42 weeks or more. Only about 57% of pregnancies reach 39–41 weeks. Preterm delivery is before 37 weeks.

How many babies are born on their exact due date?

Only about 4–5% of babies are born on their exact due date. Approximately 80% deliver within 2 weeks (before or after) their due date. The median gestational age at delivery for first-time mothers is 40 weeks and 5 days, and for subsequent pregnancies is 40 weeks and 2 days.

What factors affect the accuracy of EDD?

Factors affecting EDD accuracy include: irregular menstrual cycles (common in adolescents and perimenopause), uncertain LMP dates, recent hormonal contraception, first-trimester bleeding mistaken for LMP, obesity (affecting ultrasound accuracy), and multiple gestation (twin/triplet pregnancies tend to deliver earlier).

How is EDD calculated after IVF?

After IVF, EDD is calculated based on the date of embryo transfer and the embryo stage. For day 3 embryo transfer: EDD = transfer date + 263 days. For day 5 blastocyst transfer: EDD = transfer date + 261 days. IVF dating is more accurate than LMP-based dating because the exact date of fertilization is known.

What is the conception date estimate?

The conception date is estimated to occur approximately 14 days after the first day of LMP (for a 28-day cycle), which corresponds to ovulation. Conception date estimate = LMP + 14 days. However, sperm can survive for up to 5 days, so intercourse up to 5 days before ovulation can result in conception.

What is the earliest accurate dating ultrasound?

The earliest accurate dating ultrasound is performed at 11–13 weeks using crown-rump length (CRL) measurement. CRL at this stage provides dating accuracy within ±5–7 days. Earlier ultrasounds (6–10 weeks) can also date pregnancy but may be slightly less accurate.

What is the pregnancy wheel (gestational calculator)?

A pregnancy wheel (gestational calculator or gestational disc) is a rotating circular device used by healthcare providers to calculate EDD and gestational age. It aligns LMP date with the wheel to read EDD and provides quick reference for GA at various dates. Modern digital calculators have largely replaced physical wheels.

What is the probability of spontaneous labor at each week?

The probability of spontaneous labor at a given GA is approximately 5% at 37 weeks, 10% at 38 weeks, 15% per week at 39–40 weeks, and increases significantly after 41 weeks. The majority of nulliparous women deliver between 40 and 41 weeks.

What is the difference between estimated due date and actual delivery date?

The due date is an estimate of when the pregnancy reaches 40 weeks. Only 4–5% of women deliver exactly on this date. The actual delivery date depends on many factors including parity, cervical status, genetics, and whether labor is spontaneous or induced. The range from 37 to 42 weeks is considered normal.

What is the pregnancy length for first-time mothers?

First-time mothers (nulliparous) tend to deliver slightly later than those who have given birth before. The median GA at delivery for nulliparous women is 40 weeks and 5 days, compared to 40 weeks and 2 days for multiparous women. This difference is clinically significant.

What is the pregnancy length for subsequent pregnancies?

Subsequent pregnancies (multiparous) tend to deliver about 3–5 days earlier on average than first pregnancies. The median GA for multiparous women is approximately 40 weeks and 2 days. Labor also tends to progress faster in subsequent pregnancies.

What is the meaning of term pregnancy?

Term pregnancy is classified into categories based on GA: early term (37–38 weeks, 6 days), full term (39–40 weeks, 6 days), late term (41 weeks, 0 days to 41 weeks, 6 days), and postterm (42 weeks and beyond). The new classification was adopted by ACOG/SMFM in 2013.

What is the significance of 39 weeks for planned delivery?

Delivery at 39 weeks reduces neonatal respiratory complications (RDS, transient tachypnea) compared to delivery at 37–38 weeks. For this reason, elective induction or cesarean delivery is not recommended before 39 weeks without a medical indication. The risk of stillbirth increases after 41 weeks.

What happens if pregnancy goes past 41 weeks?

At 41–42 weeks, increased fetal surveillance is recommended (NST, BPP, AFI). Induction of labor is typically offered between 41 and 42 weeks to reduce stillbirth risk. The risk of stillbirth increases from approximately 1 per 1,000 at 40 weeks to 4–5 per 1,000 at 42 weeks.

What causes an inaccurate LMP date?

Common causes of inaccurate LMP include: irregular menstrual cycles (anovulation, PCOS), recent pregnancy or breastfeeding (delayed return of ovulation), recent oral contraceptive use (anovulation), early pregnancy bleeding misidentified as menses, and simply not remembering the exact date.

How does cycle length affect EDD calculation?

For a 28-day cycle, ovulation occurs on day 14. For a 35-day cycle, ovulation occurs around day 21. Naegele's rule assumes a 28-day cycle and may overestimate GA in women with longer cycles. The Parikh formula adjusts for actual cycle length to improve accuracy.

What is the Mittendorf-Williams formula?

The Mittendorf-Williams formula adjusts Naegele's rule based on race, parity, and maternal age. It found that white primiparous women had mean GA of 40.3 weeks, while African American primiparous women had mean GA of 39.6 weeks. The formula has not gained widespread clinical adoption.

What is the role of EDD in managing postterm pregnancy?

Accurate EDD is critical for diagnosing postterm pregnancy (42 weeks). Management includes serial NST, BPP, and AFI assessments. Induction is recommended at 41–42 weeks to reduce perinatal morbidity and mortality. The ARRIVE trial showed that elective induction at 39 weeks reduced cesarean rates.

What is the ARRIVE trial?

The ARRIVE trial (2018) randomized nulliparous women to elective induction at 39 weeks versus expectant management. It found that induction at 39 weeks reduced the cesarean delivery rate (18.6% vs 22.2%) without increasing adverse neonatal outcomes. This changed clinical practice for low-risk women.

What is the concept of physiological dating?

Physiological dating recognizes that individual pregnancy duration varies naturally, with some healthy pregnancies lasting 37 weeks and others 42 weeks. The EDD represents the population average, but individual variation is normal. The concept emphasizes that not all prolonged pregnancies are pathological.

What is the impact of maternal age on EDD accuracy?

Maternal age affects the accuracy of LMP-based dating because older women are more likely to have irregular cycles and perimenopausal anovulation. Advanced maternal age (≥35 years) is also associated with earlier delivery (mean GA 39.5 weeks vs 40.0 weeks for younger women).

How is EDD determined for twin pregnancies?

Twin pregnancies are dated using the same methods as singletons. However, the average GA at delivery is 36 weeks for dichorionic twins and 34 weeks for monochorionic twins. EDD calculation remains 40 weeks, but obstetricians plan for earlier delivery based on chorionicity and complications.

What is the role of EDD in managing pre-eclampsia?

EDD is used to determine the timing of delivery for pre-eclampsia. Delivery is recommended at 37 weeks for mild pre-eclampsia, at 34–36 weeks for severe pre-eclampsia with stable maternal/fetal status, and immediately for severe pre-eclampsia with adverse conditions regardless of GA.

What is the history of pregnancy dating?

Pregnancy dating has evolved from counting months from quickening (Hippocrates, 400 BCE) to the 40-week standard based on Naegele's mathematical calculation (1838). Later improvements include ultrasound dating (1970s–1980s), CRL nomograms, and the current ACOG dating guidelines (2022).

What is the estimated date of conception (EDC)?

The estimated date of conception (EDC) is calculated as LMP + 14 days, assuming a 28-day cycle with ovulation on day 14. The actual conception date can vary by up to 5 days due to sperm survival time (up to 5 days in the female reproductive tract). EDC is approximately 266 days before the EDD.